Magnetic powder for magnetic recording and magnetic recording medium containing the same

ABSTRACT

A magnetic powder for magnetic recording is disclosed which comprises composite magnetic particles (A), each of which contains hexagonal ferrite and spinel structure ferrite, and single phase magnetic particles of hexagonal ferrite (B). When the mixing ratio [(A):(B)] of (A) and (B) in the range from 5:95 to 95:5, remarkable effect can be obtained. Especially, when the mixing ratio of (A) and (B) is in the range from 5:95 to 30:70, the dispersion property is improved due to disturbance of magnetic stacking. Thus, the magnetic powder in this mixing ratio is particularly suitable for magnetic recording in a short wavelength range. When the mixing ratio of (A) and (B) is in the range from 70:30 to 95:5, the reproduction output is improved due to activation of (A). Thus, the magnetic powder in this mixing ratio is specifically suitable for magnetic recording in a long wavelength range. By containing the magnetic powder described above, a perpendicular magnetic recording medium with high electromagnetic characteristics, particularly a high S/N ratio and a low noise can be obtained.

This is a continuation-in-part of application Ser. No. 07/869,298, filedApr. 16, 1992, now U.S. Pat. No. 5,378,547.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a magnetic powder suitable for magneticrecording with high recording density and a magnetic recording mediumcontaining the same for example a magnetic tape.

2. Description of the Related Art

Conventionally, a magnetic recording medium of coating type is made bycoating a magnetic powder, in combination with a resin binder on anon-magnetic base material such as a polyethylene film. In recent years,as the need of high recording density of magnetic recording mediaincreases, single phase magnetic particles of hexagonal ferrite inaccordance with perpendicular magnetic recording system and magneticrecording medium using them have been developed. The magnetic recordingmedia in accordance with the perpendicular magnetic recording system aremainly made by coating a hexagonal ferrite magnetic powder such as Baferrite dispersed in a resin binder on a base material. These powdersare plate shaped and have an easily magnetizable axis perpendicular tothe plate surface. Each particle of the plate shaped hexagonal ferriteof single phase has a crystal structure where two types of layer unitsreferred to as spinel block (or S block) and R block are regularlylayered.

A magnetic recording medium using a hexagonal ferrite magnetic powder inaccordance with a perpendicular magnetic recording system can recorddata in higher density than a conventional magnetic recording mediumusing a needle shaped magnetic powder in accordance with longitudinalmagnetic recording system.

This is because the hexagonal ferrite magnetic powder is made of veryfine particles and the particles are so arranged in the magnetic layer,which is made by smoothly coating the particles on the base materialwith a high packing ratio, that magnetized direction of them areperpendicular to the medium surface and not magnetically repulsive oneanother. In addition, the magnetic recording media using the hexagonalferrite powder can provide a higher reproduction output in a shortwavelength range than the conventional medium in accordance withlongitudinal magnetic recording system.

Recently, a magnetic particle where hexagonal ferrite and spinelstructure ferrite are integrated has been proposed so as to enhance thesaturation magnetization thereof, the saturation magnetization of theformer being lower than that of the latter. This magnetic particle isreferred to as a composite magnetic particle. In the crystal structureof the composite magnetic particle, S blocks are irregularly added toother S blocks of the hexagonal ferrite, where S blocks and R blocks areregularly layered. Thereby both S blocks and R blocks are irregularlylayered in the crystal structure of this composite magnetic powder.Thus, this composite magnetic particle has been considered to possessboth the crystal structure of the spinel ferrite and that of thehexagonal ferrite.

However, thus far it was very difficult to produce a magnetic recordingmedium which, by using the above mentioned magnetic particles inaccordance with the perpendicular magnetic recording system, satisfiesall electromagnetic characteristics such as reproducing output andsignal-to-noise ratio (hereinafter, referred to as an S/N ratio).

For example, a magnetic powder of hexagonal ferrite is not easilydispersed to a resin binder in comparison with a needle shaped magneticpowder. Thus, so far the reproduction output of the magnetic recordingmedium produced by coating the magnetic powder of hexagonal ferrite isnot higher than that expected. The magnetic particles of hexagonalferrite tend to be stacked one another magnetically in the direction oftheir easily magnetizable axes which are vertical direction to thesurface of plate shaped particles. Thus, the dispersion property ofmagnetic particles of hexagonal ferrite has been considered to bedeteriorated by this magnetical stacking. As a result, it seems that themagnetic particles of hexagonal ferrite can not display their intrinsicability. In addition, since the dispersion property is deteriorated, asmooth surface of magnetic layer cannot be easily formed by usinghexagonal ferrite. Thus, the magnetic recording by using hexagonalferrite involves relatively large noise. As described above, since thehexagonal ferrite can hardly be dispersed into the resin binder, themagnetic recording medium using the single phase magnetic particlesthereof has not provided a large reproduction output and a high S/Nratio.

SUMMARY OF THE INVENTION

The present invention is made from the above mentioned point of view. Inparticular, an object of the present invention is to provide a magneticpowder for magnetic recording and a magnetic recording medium using thismagnetic powder with an improved dispersion property, thereby reducingnoise and obtaining excellent electromagnetic characteristics.

The magnetic powder in accordance with the present invention comprisescomposite magnetic particles (A), each of which contains hexagonalferrite and spinel structure ferrite, and single phase magneticparticles of hexagonal ferrite (B).

The magnetic recording medium, made by coating a magnetic powder whichis a mixture of the composite magnetic particles (A) and the singlephase magnetic particles of hexagonal ferrite (B), has a strongerresistance to noise than that made by using the single phase magneticparticles of hexagonal ferrite (B) does, thereby providing excellentelectromagnetic characteristics. In addition, the magnetic recordingmedium in accordance with the present invention can provide a higher S/Nratio in a short wavelength range than a magnetic recording medium usingthe composite magnetic particles (A) does.

When the mixing ratio [(A):(B)] of the composite magnetic particles (A)and the single phase magnetic particles of hexagonal ferrite (B) is inthe range from 5:95 to 95:5, the above mentioned effects can beremarkably obtained.

The mixing ratio in the present invention represents a weight ratio ofthe composite magnetic particles (A) and the single phase magneticparticles of hexagonal ferrite (B).

The composite magnetic particles (A) in accordance with the presentinvention are preferably a compound substantially given by one of thefollowing chemical formula (1).

    AO·n (Fe.sub.2-X-Y M(1).sub.X M(2).sub.Y O.sub.18-Z) (1)

where A is at least one element selected from the group consisting ofBa, Sr, Ca, and Pb; M(1) is at least one element selected from the groupconsisting of Co, Zn, Ni, Cu, Mn, and Fe(II); M(2) is at least oneelement selected from the group consisting of Ti, Sn, Ge, Zr, Sb, Nb, V,Ta, W, and Mo; X is a number in the range from 0.5 to 3.0; Y is a numberin the range from 0 to 2.0; Z is a number which is 0.05 or larger and isgiven by [X+(3-m)Y]/2, where m is the average valence of M(2); and n isa number in the range from 1.0 to 2.0.

The single phase magnetic particles (B) in accordance with the presentinvention are preferably a compound substantially given by the followingchemical formula (2) or (3).

    AO·Fe.sub.12-X-Y M(1).sub.X M(2).sub.Y O.sub.18   ( 2)

where A is at least one element selected from the group consisting ofBa, Sr, Ca, and Pb; M(1) is at least one element selected from the groupconsisting of Co, Zn, Ni, Mn, Cu and Fe(II); M(2) is at least oneelement selected from the group consisting of Sn, Ti, Ge, Zr, V, Nb, Ta,Sb, W, and Mo; X is a number in the range from 0.1 to 1.5; and Y is anumber in the range from 0.1 to 1.5.

    AO·M(1).sub.2 Fe.sub.16-X-Y M(2).sub.X M(3).sub.Y O.sub.26 ( 3)

where A is at least one element selected from the group consisting ofBa, Sr, Ca, and Pb; M(1) is at least one element selected from the groupconsisting of Co, Zn, Ni, Cu, Mn, and Fe(II); M(2) is at least oneelement selected from the group consisting of Co, Zn, Ni, Mn, Cu, andFe(II); M(3) is at least one element selected from the group consistingof Sn, Ti, Zr, Ge, V, Nb, Ta, Sb, W, and Mo; X is a number in the rangefrom 0.1 to 1.5; and Y is a number in the range from 0.1 to 1.5.

More preferably, the average particle diameter of the composite magneticparticles (A) and the single phase magnetic particles of hexagonalferrite (B) is in the range from 0.02 to 0.2 μm and the coercive force(Hc) thereof being in the range from 200 to 2000 Oe.

When the average particle diameter of the composite magnetic particles(A) and the single phase magnetic particles of hexagonal ferrite (B) issmaller than 0.02 μm, since the particles hardly disperse, the effect ofthe present invention cannot be obtained. In contrast, when the averageparticle diameter is larger than 0.2 μm, the reproduction performance inthe short wavelength range is deteriorated.

When the mixing ratio [(A):(B)] of the composite magnetic particles (A)and the single phase magnetic particles of hexagonal ferrite (B) inaccordance with the present invention is in the range from 5:95 to30:70, the magnetic recording medium using these particles can be morepreferably used for recording and reproducing in a wavelength range of 1μm or below.

When the mixing ratio [(A):(B)] of these composites is in the range from70:30 to 95:5, the magnetic recording medium using these particles canbe more preferably used for recording and reproducing in a wavelengthrange of over 1 μm.

As the producing methods of these magnetic particles, theglass-crystallizing method disclosed by Japanese Patent Laid-OpenPublication Serial No. SHO 56-67904, the hydro-thermal compositesintering method disclosed by Japanese Patent Laid-Open PublicationSerial No. SHO 61-168532, and so forth can be suitably used.

Then, the magnetic recording medium in accordance with the presentinvention is generally described.

The magnetic recording medium in accordance with the present inventioncan be produced by using the above mentioned magnetic powder in theconventional method that follows. The magnetic powder, resin binder, andif necessary various additives are mixed and dispersed so as to producea magnetic coating material. Thereafter, by coating the magnetic coatingmaterial on a base material, a magnetic layer is formed. Besides themagnetic powder and the binder resin, the various additives which can becontained in the magnetic layer are dispersant, lubricant, antistaticagent, and so forth. After the magnetic layer is formed, a magneticfield orientation process, a drying process, a surface smoothingprocess, and so forth are performed so as to obtain the magneticrecording medium in accordance with the present invention.

As the materials constructing the non-magnetic base material, polyestergroup such as polyethylene terephthalate (PET) and polyethylenenaphthalate and other various materials such as polyolefin group andcellulose derivatives can be used.

As the resin binder of the magnetic recording medium in accordance withthe present invention, any resin such as polyester resin, polyetherresin, polyurethane resin, and polyacryl resin, which are normally usedfor magnetic recording media, can be used. Among these materials, aresin containing a repeating unit having at least one polar groupselected from the group consisting of --SO₃ M and --OSO₃ M can be morepreferably used. (M is hydrogen or an alkali metal atom).

The reason why the electromagnetic characteristics such as S/N ratio andreproducing output of the medium produced by using the magnetic powderin accordance with the present invention are superior to those of themedium using composite magnetic particles alone or single phase magneticparticles of hexagonal ferrite alone as a magnetic powder would be asfollows.

Unlike the single phase magnetic particles of hexagonal ferrite (B), thecomposite magnetic particles (A) do not have a crystal structure withregularity, but an irregular combination of an S block of M type or of Wtype crystal structure and another S block.

On the other hand, the single phase magnetic particles of hexagonalferrite (B) have either an M type crystal structure where sets of an Sblock and an R block are regularly arranged or a W type crystallinestructure where sets of an S block, an R block and an R block areregularly arranged.

Thus, the crystal structure of the composite magnetic particles (A)differs from that of the single phase magnetic particles of hexagonalferrite (B).

Although the composite magnetic particles contain a spinel structureferrite having high saturation magnetization, the magnetic flux in theparticles tends to form a closed magnetic path through the spinelstructure ferrites which are disposed on the top and the bottom of theplate surface. This is because the spinel structure ferrites have smallmagnetic anisotropy and are easily magnetized in every direction. Thus,the magnetic flux in the particles of the composite magnetic particlesdo not contribute so much to the reproduction output of the medium.Particularly, in a short wavelength range where a demagnetization fieldworks strongly, the magnetization of the spinel ferrite on the top andbottom plane S block inclines from the direction perpendicular to theplate surface and thereby forming the closed magnetic path.

As described above, since the magnetization of the composite magneticparticles tend to form a closed magnetic path, it is supposed that themagnetic cohesive force of it is weaker than that of single phasemagnetic particle. Thus, by mixing a predetermined number of compositemagnetic particles with single phase magnetic particles of hexagonalferrite, the dispersion property of the magnetic powder of the hexagonalferrite is effectively improved and thereby providing a low noiseproperty and a high S/N ratio.

By disposing a predetermined number of single phase magnetic particlesof the hexagonal ferrite (B), which are strongly magnetized along thevertical direction to the plate surface and their magnetizationdirection is hard to incline, among the composite magnetic particles,the magnetization of the S blocks disposed on the top and the bottom ofthe plate surface of the composite magnetic particles would becomeactive. Thus, the reproduction output in accordance with the saturationmagnetization intrinsic to the composite magnetic particles could bebrought out.

The magnetic recording medium using the magnetic powder which is amixture of the composite magnetic particles (A) and the single phasemagnetic particles of hexagonal ferrite (B) shows lower noise than amagnetic recording medium using the single phase magnetic particles ofhexagonal ferrite (B) does, thereby providing excellent electromagneticcharacteristics.

Moreover, the magnetic recording medium in accordance with the presentinvention has a higher S/N ratio in a short wavelength range than amedium using the composite magnetic particles (A).

DESCRIPTION OF PREFERRED EMBODIMENTS

Embodiments in accordance with the present invention are described inthe following examples.

Embodiments 1 to 8:

First, aqueous solutions of BaCl₂, FeCl₃, CoCl₃, and TiCl₄ were preparedand mixed so that the composition of Co--Ti substituted Ba ferrite givenby the chemical formula: Ba Fe₁₂₋₂ x Co_(X) Ti_(X) O₁₉ (where x=0.6) wasobtained. Thereafter, an alkaline substance was added to the mixedsolution and then the coprecipitate including Ba, Fe, Co, and Ti atomswas precipitated at pH 13. Thereafter, the resultant mixture was heatedfor four hours at 100° C. Thus, a starting compound of the Co--Tisubstituted Ba ferrite was produced. Thereafter, equivalent molaramounts of NiCl₂ and ZnCl₂ and four times molar amount of FeCl₃ wereadded to a slurry of the starting compound which was heated to 100° C.The resultant slurry was referred to as the slurry (A).

On the other hand, aqueous solutions of BaCl₂, FeCl₃, CoCl₃, and TiCl₄were prepared and mixed so that the composition of a Co--Ti substitutedBa ferrite given by the chemical formula: Ba Fe_(12-2X) Co_(X) Ti_(X)O₁₉ (where X=0.75) was obtained. Thereafter, an alkaline substance wasadded to the mixed solution and then the coprecipitate was precipitatedat pH 13. Thereafter, the resultant mixture was heated for four hours at100° C. Thus, a starting compound of the Co--Ti substitute Ba ferritewas produced. The resultant slurry was referred to as the slurry (B).

Thereafter, mixtures 1 to 8 where the slurry (A) and the slurry (B) weremixed with various mixing rates were hydrothermally reacted and therebymixed starting compound slurries 1 to 8 were produced. Thereafter, thesemixed starting compound slurries 1 to 8 were rinsed with water untiltheir hydrogen ion concentrations became pH 8 or below.

Thereafter, the resultant mixed starting compound slurries 1 to 8 weremixed with BaCl₂ (the weight ratio of BaCl₂ to the dried mixed startingcompound was 1:1) and then satisfactorily stirred. Thereafter, theresultant mixtures were dried with a spray dryer. The resultant driedmixtures 1 to 8 were thermally processed for two hours at 900° C.Thereafter, the mixtures were rinsed with water so as to remove BaCl₂flux therefrom. Thus, magnetic powder specimens 1 to 8 which wereembodiment examples in accordance with the present invention wereproduced.

As results of X-ray diffraction of the magnetic powders thus produced,it was found that they had a mixed phase of M (magnetoplumbite) typeferrite and spinel structure ferrite. In addition, the surface of thesemagnetic powder specimens were analyzed with transmission electronmicroscopic photographs (acceleration voltage=400 kV;magnification=2,000,000 times). As a result of the analysis, it wasfound that composite magnetic particles (A) where spinel ferrite andmagnetoplumbite type ferrite were integrated and single phase magneticparticles of magnetoplumbite type ferrite (B) coexisted.

Next, separately, a single phase magnetic powder and a compositemagnetic powder were prepared by the following methods.

An alkaline substance was added to the mixture of aqueous solutions ofBaCl₂, FeCl₃, CoCl₂ and TiCl₄ with predetermined amount respectively toobtain coprecipitate, whose composition was given by the chemicalformula: BaFe_(12-2x) Co_(x) Ti_(x) O₁₉ (where x=0.75), at pH 13. Theresultant coprecipitate was heated at 100° C., and then hydrothermallyreacted at 250° C. Thereafter, the hydrothermally reacted coprecipitatewas rinsed with water until its hydrogen ion concentration became pH 8.The resultant slurry was mixed with BaCl₂ as flux, and was dried with aspray dryer. The resultant dried mixture was thermally processed for twohours at 900° C. Thereafter, the mixture was rinsed with water so as toremove BaCl₂ added as flux therefrom. Thus, magnetic powder specimen 9composed of single phase magnetic powder was produced. The single phasemagnetic powder has a saturation magnetization of 56 emu/g, a coerciveforce of 850 Oe, an average particle diameter of 43 nm and a ratio to anaspect ratio of 3.0.

An alkaline substance was added to the mixture of aqueous solutions ofBaCl₂, FeCl₃, CoCl₂ and TiCl₄ with predetermined amount respectively toobtain coprecipitate, whose composition was given by the chemicalformula: BaFe_(12-2x) Co_(x) Ti_(x) O₁₉ (where x=0.60). The resultantcoprecipitate was heated at 100° C. Thereafter, equivalent molar amountsof NiCl₂ and ZnCl₂ and four times molar amounts of FeCl₃ were added tothe coprecipitate. Then, the mixture was hydrothermally reacted at 250°C. The hydrothermally reacted mixture was rinsed with water until itshydrogen ion concentration became pH 8. The resultant slurry was mixedwith BaCl₃ as flux, and was dried with a spray drier. The resultantdried mixture was thermally processed for two hours at 900° C.Thereafter, the mixture was rinsed with water so as to remove BaCl₂added as flux therefrom. Thus, magnetic powder specimen 10 composed ofcomposite magnetic particles was produced. The composite magnetic powderhas a saturation magnetization of 65 emu/g, a coercive force of 870 Oe,an average particle diameter of 43 nm and an aspect ratio of 3.0.

Magnetic coating material 1 to 10 were produced with the magnetic powderspecimens 1 to 10 by the following method.

First, the following compositions of magnetic coating materials exceptColonate L were kneaded in a sand mill for 1 hour, 3 hours, 5 hours and10 hours respectively. In order to evaluate the dispersion of thusobtained magnetic powder, the glossiness of the films, which were coatedwith each magnetic powder and then dried, was evaluated with incidentangle of 60° according to the kneading time. The result is shown inTable 1.

                  TABLE 1    ______________________________________              Mixing              ratio  Kneading Time (hrs)              A:B    1       3       5     10    ______________________________________    magnetic coating                 5:95    113     121   129   140    material 1    magnetic coating                 7:93    120     129   135   143    material 2    magnetic coating                10:90    125     133   138   144    material 3    magnetic coating                30:70    130     136   140   145    material 4    magnetic coating                50:50    135     140   143   145    material 5    magnetic coating                70:30    139     143   145   146    material 6    magnetic coating                90:10    141     143   145   146    material 7    magnetic coating                95:5     141     145   145   146    material 8    magnetic coating                 0:100   105     111   117   133    material 9    magnetic coating                100:0    140     146   145   146    material 10    ______________________________________

Thereafter, the magnetic coating materials 1 to 8 were kneaded for 10hours as described above, and filtered through a filter with 1-μm meshesto remove aggregations. Colonate L was added to thus obtained materialsand the resultant materials were applied on PET films.

Thereafter, the surface of the PET films was smoothened by a calenderprocess. Thereafter, the resultant films were slitted in 1/2 inch width.Thus, medium specimens 1 to 8 were obtained. The magnetic powderspecimen numbers accord with the medium specimen numbers which wereproduced therewith.

    ______________________________________    Composition of magnetic coating material:    ______________________________________    Magnetic powder        100 parts by weight    Copolymer of vinyl chloride and vinyl                           10 parts by weight    acetate    Polyurethane           10 parts by weight    Lecithin                4 parts by weight    Methyl isobutyl ketone 93 parts by weight    Toluene                93 parts by weight    Colonate L (a trade name of Nippon                            3 parts by weight    Polyurethane K.K., Polyisocyanate    compound)    ______________________________________

Thereafter, to examine the properties of the medium specimens 1 to 8,their noise and S/N ratio in short and middle wavelength ranges weremeasured. To measure such properties, a ring type ferrite head with gapwidth=0.3 μm, track width=35 μm, and relative speed between head andtape =3.75 m/sec was used. The S/N ratio in the middle wavelength rangewas measured with signals having a recording wavelength of 1.0 μm. TheS/N ratio in the short wavelength range was measured with signals havinga recording wavelength of 0.4 μm. In addition, the noise of medium wasmeasured from an integrated value of a noise component in a frequencyrange from 200 kHz to 6 MHz.

Moreover, the surface roughness of the magnetic phase of each specimenwas measured.

Table 2 lists the mixing ratios of the particles and magnetic propertieswith respect to the magnetic powder specimens 1 to 8 and the evaluationresults thereof.

Comparison Example

Medium specimens 1 and 2 as comparison examples were produced in thesame manner as described above except using magnetic coating material 9and 10 instead of the magnetic coating materials 1 to 8. Thereafter, thesame evaluation as applied to the medium specimens 1 to 8 was applied tothe medium specimens 1 and 2. The result is shown in Table 2.

                                      TABLE 2    __________________________________________________________________________            Mixing                Saturation       Short Middle            ratio                magnetization                       Coercive force (Oe)                                 wavelength                                       wavelength            A:B (emu/g)                       powder                            medium                                 S/N (dB)                                       (S/N (dB)    __________________________________________________________________________    Embodiment 1             5:95                56     860  870  +1.2  +0.8    Embodiment 2             7:93                57     860  870  +1.6  +1.3    Embodiment 3            10:90                57     850  860  +2.4  +1.7    Embodiment 4            30:70                59     830  840  +2.5  +2.5    Embodiment 5            50:50                60     860  860  +1.6  +3.1    Embodiment 6            70:30                61     850  850  +1.4  +2.8    Embodiment 7            90:10                63     860  850  +1.3  +2.4    Embodiment 8            95:5                65     870  850  +1.2  +2.2    Comparison 1             0:100                56     850  860  0     0    Comparison 2            100:0                65     860  850  +1.0  +1.5    __________________________________________________________________________

Table 1 shows the magnetic coating material with larger percentage ofthe composite magnetic particles (A) than that of the single phasemagnetic particles of hexagonal ferrite (B) enables sufficientdispersion in shorter time. Table 1 also shows that effect on improvingdispersion property is saturated where the mixing ratio of the compositemagnetic particles (A) and the single phase magnetic particles ofhexagonal ferrite (B) is 70:30. Therefore, even if the ratio ofcomposite magnetic powder increases over this ratio, there cannot beseen any effect on a dispersion property.

Table 2 shows that although Comparison 2 produced with the magneticpowder specimen 10 composed of composite magnetic particles hassufficiently high saturation magnetization, for S/N ratio, eachEmbodiment was superior to Comparison 2.

Considering the consequences mentioned above, magnetic powder consistingof both the composite magnetic particles (A) and the single phasemagnetic particles of hexagonal ferrite (B) not only require less timefor dispersion but also has a higher S/N ratio than a magnetic powderconsisting of either only the composite magnetic particles (A) or onlythe single phase magnetic particles of hexagonal ferrite (B).

Among the magnetic powder whose mixing ratio of the composite magneticparticles (A) and the single phase magnetic particles of hexagonalferrite (B) is in the range from 5:95 to 95:5, that with a mixing rationin the range from 70:30 to 95:5 accomplishes particularly excellenteffect.

As shown in Table 2, since the magnetic powder for magnetic recording inaccordance with the present invention comprises composite magneticparticles (A) and single phase magnetic particles of hexagonal ferrite(B), the magnetic powder has a higher dispersion property than amagnetic powder for magnetic recording using only the single phasemagnetic particles of hexagonal ferrite (B), thereby reducing noise andproviding an excellent S/N ratio both in a short wavelength range and amiddle wavelength range.

Since the S/N ratio in the short wavelength range of the magneticrecording medium only using the composite magnetic particles (A) as amagnetic powder for magnetic recording is +1.0 (dB), it is found thatthis is an excellent effect accomplished by the present invention.

In addition, when the mixing ratio of the composite magnetic particles(A) and the single phase magnetic particles of hexagonal ferrite (B) isin the range from 30:70 to 95:5, since an excellent dispersion propertycan be obtained, the surface property of the magnetic phase of therecording medium is improved, thereby reducing noise.

In the magnetic recording techniques, when a reproducing output isamplified, noise component is also amplified. Thus, to obtain a high S/Nratio, it is necessary to decrease the noise component. When the mixingratio of the composite magnetic particles (A) and the single phasemagnetic particles of hexagonal ferrite (B) is in the range from 70:30to 95:5, S/N ratio and the dispersion of the magnetic powder werepreferably improved.

In consideration of a wavelength range of over 1 μm for use with colorsignals of video tapes, when the above mentioned mixing ratio was in therange from 70:30 to 95:5, S/N ratio was preferably improved.

For example, when a signal was recorded with a recording wavelength of 5μm on the media of the embodiments 2 and 7, their S/N ratios were +1.2and +4.4 (dB) respectively, while the S/N ratio of the medium of thecomparison example was 0.

On the other hand, in recording and reproducing a signal in a shortwavelength range of 1 μm or below, when the mixing ratio of thecomposite magnetic particles (A) and the single phase magnetic particlesof hexagonal ferrite (B) were in the range from 5:95 to 30:70, anexcellent S/N ratio and the dispersion of the magnetic powder could beobtained. This S/N ratio is most suitable for high density digitalrecording.

As was described above, according to the present invention, whencomposite type ferrite magnetic particles (A) coexist with single phasemagnetic particles of hexagonal ferrite (B), the dispersion property ofthe hexagonal ferrite magnetic powder (B) can be improved. Thus, themagnetic powder which can be used for production of a magnetic recordingmedium which reduces noise and has a high S/N ratio and a highreproducing output can be obtained.

What is claimed is:
 1. A magnetic powder for magnetic recording, comprising a mixture of:composite magnetic particles (A), each of said particles (A) containing hexagonal ferrite and spinel structure ferrite; and single phase magnetic particles of hexagonal ferrite (B), wherein the average particle diameter of said composite magnetic particles (A) is in the range from 0.02 to 0.2 μm; and the average particle diameter of said single phase magnetic particles of hexagonal ferrite (B) is in the range from 0.02 to 0.2 μm.
 2. A magnetic powder for magnetic recording according to claim 1, wherein the mixing ratio (A):(B) of said composite magnetic particles (A) and said single phase magnetic particles of hexagonal ferrite (B) is in the range from 5:95 to 95:5.
 3. A magnetic powder for magnetic recording according to claim 2, wherein said composite magnetic particles (A) is a ferrite substantially given by the chemical formula (1);

    AO·n (Fe.sub.12-X-Y M(1).sub.X M(2).sub.Y O.sub.18-Z) (1)

where A is at least one element selected from the group consisting of Ba, Sr, Ca, and Pb; M(1) is at least one element selected from the group consisting of Co, Zn, Ni, Cu, Mn, and Fe(II); M(2) is at least one element selected from the group consisting of Ti, Ge, Sn, Sb, Nb, V, Zr, W and Mo; x is a number in the range from 0.5 to 3.0; Y is a number in the range from 0 to 2.0; Z is a number which is 0.05 or larger and is given by [X+(3-m) Y]/2, where m is the average valence of M(2); and n is a number in the range from 1.0 to 2.0.
 4. A magnetic powder for magnetic recording according to claim 2, wherein said single phase magnetic particles of hexagonal ferrite (B) is a ferrite substantially given by the chemical formula (2) or (3);

    AO·Fe.sub.12-X-Y M(1).sub.X M(2).sub.Y O.sub.18   ( 2)

where A is at least one element selected from the group consisting of Ba, Sr, Ca, and Pb; M(1) is at least one element selected from the group consisting of Co, Zn, Ni, Mn, and Cu; M(2) is at least one element selected from the group consisting of Sn, Ti, Ge, V, Nb, Ta, Sb, W, and Mo; X is a number in the range from 0.1 to 1.5; and Y is a number in the range from 0.1 to 1.5;

    AO·M(1).sub.2 Fe.sub.16-X-Y M(2).sub.X M(3).sub.Y O.sub.26 ( 3)

where A is at least one element selected from the group consisting of Ba, Sr, Ca, and Pb; M(1) is at least one element selected from the group consisting of Co, Zn, Ni, Cu, Fe, Mn, and Fe(II); M(2) is at least one element selected from the group consisting of Co, Zn, Ni, Mn, Cu, and Fe(II); M(3) is at least one element selected from the group consisting of Sn, Ti, Zr, Ge, V, Nb, Ta, Sb, W, and Mo; X is a number in the range from 0.1 to 1.5; and Y is a number in the range from 0.1 to 1.5.
 5. A magnetic powder for magnetic recording according to claim 1, wherein the mixing ratio (A):(B) of said composite magnetic particles (A) and said single phase magnetic particles of hexagonal ferrite (B) is in the range from 5:95 to 30:70.
 6. A magnetic powder for magnetic recording according to claim 1, wherein the mixing ratio (A):(B) of said composite magnetic particles (A) and said single phase magnetic particles of hexagonal ferrite (B) is in the range from 70:30 to 95:5.
 7. The magnetic powder for magnetic recording according to claim 1, wherein the coercive force (Hc) of said composite magnetic particles (A) is in the range from 200 to 2000 Oe; and the coercive force (Hc) of said single phase magnetic particles of hexagonal ferrite is in the range from 200 to 2000 Oe. 